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I didn't know that, Stonks. I put my disturbed sleep pattern down to "the change". I've just had a quick look into Vitamin D deficiency and it's quite common in people with Crohn's disease (like me) and can cause other symptoms like pain and weakness in the joints - which I also have, but had put down to getting older. Maybe I should ask the doc to check it out - although some doctors get stroppy if they think you are self-diagnosing.
Just ask Yogi, its your body, I have a stroppy doctor but I've learnt to be as stroppy as him, it was me that pointed it out to my specialist that I thought I suffered from SAD, he was shocked how low my vitamin D levels came back and I take vitamins everyday and since having my injections I don't get winter depression either..